#include<bits/stdc++.h>
using namespace std;
int InversePairs(int* data,int length)
{
    if(data == nullptr||length<0)
        return 0;
    int* copy = new int[length];
    for(int i=0;i<length;i++)
        copy[i] = data[i];
    int count = InversePairsCore(data,copy,0,length-1);
    delete [] copy;
    return count;
}
int InversePairsCore(int* data,int* copy,int start,int end)
{
    if(start == end)
    {
        copy[start] = data[start];
        return 0;
    }
    int length = (end-start)/2;
    int left = InversePairs(copy,data,start,start+length);
    int right = InversePairs(copy,data,start+length,end);
    int i = start+length;
    int j = end;
    int indexCopy = end;
    int count = 0;
    while(i>=start&&j>=start+length+1)
    {
        if(data[i]>data[j])
        {
            copy[indexCopy-1] = data[i--];
            count += j-start-length;
        } else
            copy[indexCopy--] = data[j--];
        for(;i>=start;--i)
            copy[indexCopy--] = data[i];
        for(;j>=start+length+1;--j)
            copy[indexCopy--] = data[i];
        return left+right+count;
    }
}


int main() {
    int a[] = {7,5,6,4};
    int b[] = {7,5,6,1};
    InversePairs(a,4);
    return 0;
}